Mineral Sequestration: Storing Carbon Dioxide as Rock

There’s an old saying geologists like to bring up from time to time, and it goes something like this: “Sticks and stones may break my bones, but the way to solve my greenhouse gas problem is to inject carbon dioxide into rock formations where it can react to form stable minerals over time.” Ok, so maybe that’s not a saying. But the idea is definitely worth a thought.

Carbon capture and sequestration (CCS – sequestration being just a fancy word for storage), is an idea that’s been around for a couple of decades. It involves three processes – capture, transportation, and storage. Capture requires a concentrated source of CO2 such as a coal-fired power plant or an oil refinery. Most power plants today (at least in developed nations) are equipped with ‘scrubbers‘ to remove sulphur from flue gas (if released, sulphur reacts with water in the atmosphere and forms acid rain). Similarly, plants can be retrofitted with a scrubber which removes CO2. Recently, researchers have also proposed technologies to remove CO2 directly from the air, but these technologies are still a long way from being implemented on a large scale.

Once CO2 is captured, it is transported by pipeline to the injection location. Pipelines are expensive to build and their construction has large land use and environmental impacts; thus, it makes sense to design CCS projects to minimize the distance that CO2 must be transported. At the injection location, CO2 is pushed deep beneath the ground. In a traditional storage scenario, CO2 is injected into a high porosity aquifer such as sandstone. Geologists identify formations with enough empty space (pore space) between grains to act as a sponge and store large volumes of CO2. The aquifer (reservoir) must be capped by a low porosity ‘seal‘ to trap CO2 and prevent it from escaping. A good example is Alberta’s Shell Quest project, currently under construction. Once completed in 2015, it will capture and store 1.2 million tons of carbon dioxide 2 km (1.3 mi) below the ground.

A few recent research projects have proposed a slightly different version of storage. Instead of high porosity aquifers, CO2 is injected into lower porosity rocks such as basalt and peridotite. The chemistry of these rocks is such that CO2 will react over time to form stable minerals. Basalt, just like sandstone, is incredibly common. Therefore the total volume of ‘storage space’ worldwide far exceeds all the emissions we have released into the atmosphere in the past. As always though, there is a catch. CCS comes with quite the price tag – roughly $50 to 60 per ton of CO2. Most of this cost comes from capture, and research is looking into more efficient and cost-effective scrubber technologies. That said, CCS technologies are well-developed and can easily be deployed on large scales as needed.

Be warned: you may have just sustained a lethal dose of mostly harmless science. 

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Cover photo courtesy of EnvironmentBlog, Flickr Creative Commons.